Witnesses say an explosion has gone off in Kenya's capital, the same day a grenade attack at a Nairobi bar wounded at least 13 people.

Initial reports say the explosion occurred Monday evening at a Nairobi bus stop, killed at least one person, and injured several others.

Earlier Monday, an unidentified assailant threw a grenade into a downtown Nairobi bar. Police are investigating the attack, which followed warnings from Somali militant group al-Shabab that it would launch attacks inside Kenya.

Last week, Kenya sent troops into Somalia in pursuit of the militants, who Kenyan officials blame for the cross-border kidnapping of several foreigners.

Police say the Russian-made grenade used in the bar attack was of the same type used last year in an attack on a Nairobi bus headed to Kampala. Investigators in Kenya and Uganda have linked that attack to al-Shabab.

Two days ago, the U.S. embassy in Kenya warned of an imminent terror attack in the country, although it did not specify who would carry it out. The U.S. said the terrorists would likely target places where foreigners gather, such as shopping malls and nightclubs.